Many spontaneous B-cell lymphomas in NFSv+ congenic mice, and several cell lines and/or frozen cells derived from these tumors (see 00284-18; 00286-18), put us in a unique position to study various cellular genes involved in the development of mouse B-cell lymphomas. Tumor DNAs were analyzed by Southern blotting for lineage determination, i.e. T or B. For precise determination of new ecotropic proviral integration, each tumor DNA was compared with the respective tail DNA from the same mouse. In many cases, multiple restriction endonuclease digestions of DNAs were needed for precise determination of the number of newly acquired proviruses or the absence of such integrations. All lymphomas tested contained clonal populations, the majority being monoclonal, and 87% had detectable new somatic integration of ecotropic MuLV. In tests with 4 probes, these tumors rarely exhibited rearrangement of cellular genes previously described as abnormal in T cell (c-myc, Fis-1, Pim-1, Gfi-1) and B cell (Gfi-1, Pim-1) tumors. We have obtained seven molecular clones of the DNA regions flanking new MuLV integrations. Six of the seven integration sites are present in the primary B cell lymphoma, as well as in cell lines derived from them. Sequencing of 14 host viral junction fragments and repeated ?BLAST? searches revealed that these are hitherto unidentified sequences. To date, unique sequence probes from both 5? and 3? cellular sequences from one such clone have been obtained and others are in progress. Screening of 200 tumor DNAs encountered no other rearrangement so far. This DNA sequence maps in the middle of mouse chromosome 5.Swiss mice are useful for studies of retrovirus induced diseases and carcinogenesis investigations because they are less prone to various diseases. Based on extensive studies of NIH Swiss and NFS mice, it was thought that these mice were negative for ecotropic virus, a factor which also eliminated the recombination necessary for MCF virus formation and possibly eliminating certain types of lymphoma development. Recently we analyzed three Swiss-derived strains ? CFW (see also 00284 and 00286), CD-1 and CF-1. CFW, CD-1 and CF-1 mice have either germline transmitted or somatically acquired ecotropic viral genome(s). Virus was detected where there was no detectable copy, suggesting maternal transmission and no virus was detected in other animals with multiple copies suggesting a defective or a silent nature of these genomes. One ecotropic isolate from a CFW mouse has been analyzed using restriction endonuclease mapping and was found to be identical to Akv-derived virus. A ecotropic mouse virus (Emv) locus study did not find a match with the loci of inbred strains. - Murine B cell lymphoma; ecotropic MuLV; retrovirus; molecular cloning